Natural processes including changes in solar output, plant evolution, the gradual drift of the continents, slow changes in ocean currents, and long-lasting volcanic eruptions have led to past climate changes over timescales of millions of years. Changes in atmospheric carbon dioxide levels through photosynthesis or chemical weathering have often played an important part.

Scientists think there were periods over 600 million years ago when almost the entire Earth was covered in ice. These ‘Snowball Earths’ may have been caused by low solar intensity, continental configurations, reduced carbon dioxide (CO2) levels or a combination of factors. There is no certainty about this because reconstructing such ancient climates is extremely difficult. Some evidence suggests that after the ice had spread, large amounts of CO2 – possibly from volcanic eruptions – accumulated in the Earth’s atmosphere. High CO2 is usually associated with high temperatures, but CO2 doesn’t generate heat – it only traps heat. If almost all sunlight was reflected by ice, the Earth wouldn’t give off much heat for CO2 to absorb.

Nearly 400 million years ago, levels of carbon dioxide (CO2) in the atmosphere were higher than today and global temperature was warmer. Over millions of years, many new plant species evolved. The plants absorbed CO2
from the atmosphere through photosynthesis, storing the carbon and releasing the oxygen back into the atmosphere. The resulting high oxygen levels caused the size of animals to increase – fossils show metre-long scorpions and seagull-sized dragonflies. Because CO2
is a greenhouse gas, the reduction in CO2
cooled the Earth down. Scientists estimate that global temperature fell about 10 °C over about 100 million years, from around 375 to 275 million years ago – known as the Carboniferous period.

Today most volcanoes erupt briefly with explosive force, ejecting ash and debris which blocks out sunlight and cools the Earth. But some volcanoes – flood basalts – spew out lava continually for a long time. In one example, about 251 million years ago, a flood basalt eruption lasted about a million years and covered an area nearly the size of Europe in lava, forming a large expanse of volcanic rock called the Siberian Traps. As lava spreads out, it releases trapped gases, including carbon dioxide. The Siberian Traps lava emitted huge amounts of carbon dioxide which stayed in the atmosphere for some time after the eruption ended. Scientists think this increased global temperatures by about 6 °C over a couple of million years.

The continents drift around the Earth, moving about 3 cm per year. Scientists calculate that around 50 million years ago the Indian subcontinent ploughed into Asia, gradually forming the Himalayan mountain range. The exposure of large rock surfaces to the air caused an increase in chemical weathering – which removes carbon dioxide from the atmosphere – cooling the planet. The rise of this towering mountain range also encouraged glacier formation, since temperatures decrease with altitude in the lower atmosphere. White ice and snow spreading across darker land cooled the planet by reflecting more sunlight. The Himalayas are still rising, affecting atmospheric circulation patterns and influencing the monsoons.

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The slow drift of the continents can affect the climate by altering the flow of ocean currents, which carry heat around the globe. The presence or absence of land at the poles also influences the accumulation of polar ice, since ice forms more readily on land than on water. Around 33 million years ago Antarctica broke away from Australia and South America, leaving the Southern Ocean in-between. Over time, the presence of a large land mass at the South Pole, isolated by a cold ocean current flowing around it, enabled the Antarctic ice sheet to form. Scientists estimate that temperatures dropped by about 5 °C over tens of thousands of years.

Around three million years ago – during a period known as the Pliocene – atmospheric carbon dioxide (CO2) levels were higher than today and the Earth was warmer. In addition to increased atmospheric CO2, scientists think stronger ocean currents also contributed to the warmer conditions by carrying more heat to the poles, which melted some polar ice. Positive feedbacks also played an important part – ice melt leads to less sunlight being reflected and more being absorbed, magnifying the warming. This period saw the evolution of some of our first human-like ancestors who walked on two legs. Scientists have estimated that temperatures increased about 2–3 °C over hundreds of thousands of years before falling again as CO2 levels decreased.

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Jane Francis is a palaeoclimatologist who studies past climates. Her research shows that the Earth was warm enough for forests to grow in polar regions millions of years ago. ‘This research can help us understand how our climate might change in future as our world gets warmer.’ Spending her time at Leeds University and out in the field in Antarctica, Jane collects fossils to understand what vegetation was growing millions of years ago. ‘Using these fossils I’ve been able to reconstruct what polar forests looked like millions of years ago – a bit like a detective, piecing the story together from bits of evidence.’

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There are many institutions and organisations around the world researching climate science, how our climate is changing, and ways of responding. Here are just a few…

British Antarctic Survey (BAS)

Department for the Environment, Food and Rural Affairs (Defra)

Department of Energy and Climate Change (DECC)

Energy Saving Trust (EST)

Environmental Change Institute (ECI)

European Space Agency (ESA)

The Geological Society (GS)

Grantham Institute for Climate Change (GICC)

Intergovernmental Panel on Climate Change (IPCC)

Met Office (MO)

National Academy of Sciences (NAS)

National Aeronautics and Space Administration (NASA)

National Oceanic and Atmospheric Administration (NOAA)

National Oceanography Centre (NOC)

The Royal Society (RS)

Tyndall Centre for Climate Change Research (TCCCR)

UK Climate Impacts Programme (UKCIP)

United Nations Framework Convention on Climate Change (UNFCCC)

World Climate Research Programme (WCRP)

World Meteorological Organization (WMO)

Climate change

A change in the climate that can occur over various time and spatial scales, including globally. Climate change has occurred naturally throughout the Earth’s history, with numerous causes such as changes in the Earth’s orbit, fluctuations in ocean circulations and variations in the Sun’s activity.

Over recent decades, the majority of climate scientists have concluded that the current period of global climate change is real and that human activities are the main driver.

Carbon dioxide

An important greenhouse gas, with the chemical formula CO2. After water vapour, carbon dioxide is the biggest contributor to the greenhouse effect.

Photosynthesis

A chemical reaction in plants which uses light from the Sun to help turn carbon dioxide and water into sugars, which plants then use as ‘food’.

Chemical weathering

The breakdown of rocks through chemical reactions. Normally the minerals within rocks react with rainwater, which is slightly acidic. The process of chemical weathering removes carbon dioxide from the air and stores it in rocks.

Climate

A summary of the weather in a particular region over a period of at least ten years, but more commonly defined over 20 - 30 years. The climate describes both the average weather conditions (for example temperature, rain, snow and wind) in a particular region as well as the extremes.

Lava

Hot, melted rock that has been ejected by volcanic activity onto the Earth’s surface. It differs from magma, which is hot, melted rock below the Earth’s surface.

Flood basalt eruption

A massive ejection of magma from a volcanic eruption that can continue for many thousands of years, covering large stretches of land or the ocean floor, and releasing immense amounts of gases into the atmosphere.

Antarctic

The area around the Earth’s South Pole, contained within the Antarctic Circle (a circle of latitude drawn around the southern part of the globe). It includes part of the Southern Ocean and the continent of Antarctica, a land mass covered in ice several kilometres thick.

Feedback

A process with two interconnected factors whereby the first factor affects the second, and the change in the second factor in turn affects the first. This can either increase the rate of the process (positive feedback) or decrease it (negative feedback).

An example of a positive feedback process is the melting of ice floating on an ocean. When the local temperature increases it melts the ice and reveals the dark ocean underneath. The ocean absorbs more of the Sun’s energy than the ice, causing the local temperature to increase further and leading to even more melting.